flush an objects cache when mixin in

... if this object already is an mixin. Otherwise when mixin in
several things in a row, it would collect precomputed NFAs and
this would result in bigger files when serializing it.
Example: declare 10 infixes, was 9MB .pir file, is now 1.2MB.
         compiling it to pbc took 49s now takes 0.8s.

src/Perl6/Metamodel/Mixins.nqp

@@ -2,8 +2,11 @@ role Perl6::Metamodel::Mixins {
     has $!is_mixin;
     method set_is_mixin($obj) { $!is_mixin := 1 }
     method is_mixin($obj) { $!is_mixin }
+    method flush_cache($obj) { <...> }
 
     method mixin($obj, *@roles) {
+        # Flush its cache as promised, otherwise outdated NFAs will stick around.
+        self.flush_cache($obj) if !nqp::isnull($obj) || self.is_mixin($obj);
         # Work out a type name for the post-mixed-in role.
         my @role_names;
         for @roles { @role_names.push($_.HOW.name($_)) }